It is possible at some times of the year to field dry wet coals in certain areas of the country. However, even in the most arid sections of the United States, field drying is unreliable and consumes a great amount of time and energy for such tasks as loading and unloading the coal off the ground or drying area. The prior art is replete with many methods and apparatus for drying wet coal. One such method is disclosed in Verschuur et al U.S. Pat. No. 3,992,784, issued in 1976, for a process for upgrading wet brown coal by heating at a temperature of at least 150.degree. C. and at a pressure that is higher than the vapor pressure of water at that temperature. The amount of bound water in the patentees' coal, even in a slurry, depends on a higher temperature during treating, i.e. the higher the temperature the more water that will be withdrawn from the coal particles.
A process for stabilizing dried coal was disclosed in Norman et al, U.S. Pat. No. 4,043,763 (1977), which comprises mixing as-mined lignite and sub-bituminous coal with hot, either completely or partially dried lignite or sub-bituminous coal in an amount to produce a weight ratio of dried coal to as-mined coal of from about 1:2 to about 10:1. Another method of in-situ coal drying was disclosed in Seitzer U.S. Pat. No. 4,213,752 (1980) comprising passing wet low-rank coal at room temperature into a moving bed of hot coal at a temperature in the range of 200.degree. to 300.degree. C. at a rate sufficient to maintain partial combustion of the coal in the presence of a gas, containing at atmospheric pressure, 5 to 25% by volume oxygen. Both of these methods are energy intensive. The former uses coal having a moisture level of not more than 10%, while in the latter hot coal must be maintained at 200.degree. to 300.degree. C. for its downstream admixture with the wet low-rank coal. Another process for upgrading lignitic-type coal is taught in Koppelman, U.S. Pat. No. 4,052,168 (1977), wherein the as-mined coal is subjected to an autoclaving treatment. This, of course, is a capital intensive process since pressure vessels are needed.
Many apparatus have been devised for permitting the contact of hot combustion gases with wet coal particles. Some of these apparatus concern spiral or rotary type dryers while others comprise various modifications of these units as well as upright fluid bed contacting systems. As early as 1914 a patent issued to Bornmann U.S. Pat. No. 1,102,714 teaching the passage of heated gas in a cocurrent manner with the direction of flow of wet coal wherein the apparatus has a baffle to direct the flow of the coal particles through the dryer before their withdrawal through a bottom outlet. The patentee teaches that by the adjustment of the thickness of the layer of coal via the baffle the saturation of the drying air can be correspondingly and accurately adjusted. Twenty seven years later, in 1941, U.S. Pat. No. 2,227,634 issued to Dalin disclosive of a method of conditioning grain. A drying coil is provided for removing moisture (if it is excessive) in the top part of the holding tank, which is fed through the bottom of a truncated funnel area via a moveable gate or valve in the bottom of same.
U.S. Pat. No. 4,401,436, issued to Bonnecaze, in 1983 teaches a process for drying of coal by taking advantage of the combustion of coal fines. After drying, but before cooling, the coal is oxidized in a special oxidizing vessel as shown in FIG. 3 of that patent. Air, which is the most economically efficient oxidizing agent, can be added to oxidize the coal by means of a special air distribution system which is further exemplified in FIG. 5. Also in 1983, U.S. Pat. No. 4,389,796 issued to Odman for a heat exchanger wherein heated air and particulate material both pass through the top of a drying vessel cocurrently after which the heated air egresses downwardly into a bed of the particulate material and then flows in an upward direction through the particulate material causing the desired heat exchange. The cool gas will contact the hot particulate materials thereby warming the former and cooling the latter. The cooled particulate material flows in a downward direction through a conical intermediate shape and exits a truncated portion of the conical vessel via a tubular upper part.
The instant invention uses an elongated conduit which is positioned horizontally with respect to the flow of wet coal as opposed to Odman's vertical orientation. Also, the wet particulate coal is added to a more or less even pile and not to the top of an annular conveyer. In fact, the method of addition of Odman will pose problems of flowability of the coal due to the constricted inlet.
The open air outlet of the patentee's gas inlet tube will not and could not function to evenly distribute air evenly along the length of the dryer, i.e. a perforated outlet is necessary on the horizontal gas inlet tube to perfect proper admission of the air supply to the falling wet coal. For example, if the elongated air supply tube used only a simple opening at the bottom of the horizontal tube then nearly all the gas would exit the horizontal tube at a point juxtaposed to the inlet manifold.
The patentee has a spent air outlet within the confines of his vessel. The invention of this application permits the spent air, after drying, to egress from the top of the V-shaped trench into the ambient atmosphere, although the discharged vapors are subject to appropriate scrubbing by means of a collecting hood. Thus, in the instant apparatus, exit velocities are low and dust entrainment is minimized which are advantageous.
A big advantage of this invention is that pressure drop problems in the apparatus are greatly minimized. In the patentee's apparatus the outlet area of the gas is greatly restricted since it must pass up through the annular tube to the point of discharge. This causes high gas velocities in the area and high pressure drop. The instant apparatus provides for continually expanding areas for gas flow once same exits the elongated air supply tube. This reduces pressure drop.
Apparatus generally disclosed for contacting solids with gases, such as Odman, are classified in many areas of technology in the Patent Office Classification Manual, i.e. Classes 34 and 422. In the applicable subclasses of Class 34, Gray U.S. Pat. No. 3,339,287 and McClure U.S. Pat. No. 2,671,057 are disclosive of gas-solid contacting apparatus. In the latter, solids flow through an inlet at the top and an outlet at the bottom and intermittently contact gas through a series of louvers situated in communication with a central gas manifold. In this manner, the particles moving downward are contacted with a nearly radial flow of gaseous material. A series of interrelated baffles act as a gas egress means and form one dimension of the downward moving solid particulate material (catalyst). A conduit is provided in the bottom of McClure for the augmentation of a stripping media to the solid particulate matter to remove any hydrocarbonaceous material entrained therein.
In the former patent free flowing material contacts a rising gas supply. A multitude of formed cells are provided for drying downwardly passing solid material via upwardly flowing hot gases. The contact is increased as material falls from one unit through to another. Electric elements enhance drying by boiling water off the material being dried.
Air is simply not forced through the descending material. The multiple troughs present structural problems due to sheer size. The continuous dropping of coal particles through the multiple troughs would create severe dust problems not presented by the instant apparatus. In fact, the latter even serves as a granular-type filter media to prevent, not encourage, dust particles from becoming airborne.
All of the above references, even though replete with different techniques for contacting wet particulate material with hot dry gas, have failed to disclose or teach an apparatus for shallow slot drying of wet coal in a trench in a manner such that the temperature of the hot gaseous medium can be regulated to mitigate problems of spontaneous combustion and dust while maximizing the heat exchange coefficient of the heated gas upon the coal particulate material. In this manner, the instant slot drying apparatus does not necessitate or require any cooling step for the coal egressing from the relatively dry outlet while mitigating problems of pressure drop of the drying air passing upwardly through the countercurrent flow of wet coal.